Which is the best motor type for this application?

[cicciobaruffi]

Hello,
I need to build a test instrumentation for a customer and I would like to ask your feedback regarding the best motor type to be used.
The system will be made like this:

The goal is to actuate a torque profile in a way that the force acted on the rope by the device is opposed.
The requirements are:
1- The control needs to be with torque set point (I need to create a torque profile)
2- The torque required ON THE MOTOR is from 1Nm to 4 Nm and I can not decrease it adapting the mechanical transmission
3- The motor will actuate this torque very often with 0 speed
4- The power supply will be 230 VAC 1-phase and, if required, I can use a Vac-Vdc converter
5- The cost of the equiment must be as low as possible

If I didn’t have any cost costraints my choice would be AC brushless motor + servo drive and by this solution I could satisfy all the requirements but, unfortunately, it is absolutely too expensive for the customer.
So I am searching for cheaper solution. I searched for DC motors and AC motors but I don’t have a lot of experience with this kind of motors and I ask your help.


Can you please give me your feedback about it?
Please write me if something is not clear and thanks in advance!

Thanks!

 

[Strong]

Why not use an available electric winch or hoist?

Walt

 

[cicciobaruffi]

Thanks Walt for the reply!

I searched about winch and hoist but it seems that is not possible to define torque profile (not fixed torque) or to change (by software algorythm) the desired torque while the test is in progress..do you know if it is possible with these devices?

Thanks

 

[waross]

Our VFD Gurus may be taking the day off. Wait until tomorrow.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TugboatEng]

Since force seems to be your most important criteria and force control in electric motors requires expensive feedback loops involving shaft encoders and drives, perhaps pneumatics are a better choice. A simple I-P converter and a pneumatic motor can give you variable force control in an open loop system with no risk of overheating when stalled.

 

[BrianE22]

Where are the VFD guys? I'll offer my ideas in the meantime. Torque is roughly proportional to current and it is possible to control current in motors. Certainly servo motors can do it and I believe VFDs and 3 phase AC motors can do it. How accurate are your needs for torque (or force) control?

 

[3DDave]

The VFD guys have noted that the desired price is -free- and there are no solutions close to that.

 

[djs]

Perhaps a shunt wound DC motor would work. The output torque of such a motor is linearly related to the current consumed by the armature. To control the torque, you control the armature current. Probably be accurate to better than 5%.

 

[Compositepro]

Using torque control of a motor usually results in very poor control of force on the other side of a gearbox. This is due to unknown friction in the gearbox that varies with load and speed. Thus you need a load cell to measure the actual force and feed the information back to a controller. This gets pretty expensive.

Cheaper ways to control force are weights, levers, springs, pneumatics or hydraulics.

 

[waross]

Plain induction motor driving through a magnetic clutch. For fine control use a dancer arm.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TugboatEng]

Is heating of the clutch an issue or not really at that low of a power level?

 

[BrianE22]

Something to look at would be vector control drives. I believe they do well controlling torque at low (and zero) speeds.

 

[jraef]

Well, as a "VFD guy", the problem I see here is that at 4Nm maximum torque at the motor, that comes to about 0.6kW on a 4 pole 50Hz AC induction motor. Can it be done? Sure, but a motor even that small, with a separately powered blower (because it may not actually "spin" to cool itself), plus an encoder, plus a VFD capable of true Flux Vector Control and intelligent enough to make decisions will likely cost, in US$, close to $2000. It didn't sound as though this is a tenable solution. A small servo would likely cost less actually.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Hi Tugboat. I haven't seen a variable speed magnetic clutch for decades now. They were used for speed control in some applications. I don't remember heating issues. The clutch that I was responsible for ran fairly fast. Lots of air movement for cooling and less heat generated than when running at slower speeds.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cicciobaruffi]

Hello guys,
thanks to everyone for the reply.

Customer would like to avoid the usage of pneumatic air and now the possible solutions are:
1- VFD with flux control: I agree with jraef and I think that this solution is feasible but not extremely cheap
2- Servo Motor: from my point of view (except for the price) this would be the best solution and I could really use torque profiles with closed loop (with the torque on the motor shaft).
3- DC Motor: I never used one but probably this solution would be the cheapest.

In all these solutions the problem said by Compositepro remains and the way to decrease it would be to find a way to decrease the gear ratio or to find a smart mechanical transmission.


I have two more questions: speaking about the DC motor torque control. In your experience, the performance of the torque control are similar to the servo ones? And, moreover, considering that the motor will often actuate the torque at zero speed, will it damage the motor on the long run?

Thanks!

 

[itsmoked]

will often actuate the torque at zero speed, will it damage the motor on the long run

Depends. Most servo motors can run motionless continuously without frying. When you reach certain power levels and sizes that breaks down and they start limiting you to duty cycles.

Keith Cress
kcress -

http://www.flaminsystems.com